1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a LCD device capable of preventing an inferiority thereof due to a signal lowering by increasing a set pulse width of a scan signal applied to a gate line.
2. Description of the Related Art
A liquid crystal display (LCD) device is a transparent flat panel display device, and is being widely applied to each kind of electronic device such as a mobile phone, a PDA, a notebook computer, etc. Since the LCD device has light, thin, short and small characteristics and can implement a high picture quality, it is being used more than other flat panel display devices. Moreover, as a demand for a digital TV, a high picture quality TV, a wall mounted TV is increased, a large LCD to be applied to the TVs is being researched more actively.
The LCD device is divided into several devices according to a method for driving liquid crystal molecules. Among the several devices, an active matrix thin film transistor LCD device is being mainly used due to a fast response time and less residual image.
FIG. 1 is a view showing a structure of a panel of the TFT LCD. As shown, a plurality of gate lines 3 and data lines 5 arranged horizontally and vertically for defining a plurality of pixels are formed on the liquid crystal panel 1. A thin film transistor acting as a switching device is arranged in each pixel and is switched when a scan signal is sent to the pixel through the gate line 3 thereby to apply an image signal sent through the data line 5 to a liquid crystal layer 9. The reference numeral 11 denotes a storage capacitor for sustaining a data signal received until the next scan signal is sent to the pixel.
A scan signal is applied to the gate line 3 from a gate driving unit 20, and an image signal is applied to the data line 5 from a data driving unit 34. Generally, the gate driving unit 20 and the data driving unit 34 are formed of a driver integrated circuit (IC) and arranged outside the liquid crystal panel 1. However, recently, an LCD device in which the gate driving unit 20 is integrally formed at the liquid crystal panel is being actively researched. As the gate driving unit 20 is integrally formed at the liquid crystal panel 1, the LCD device has a decreased volume and fabrication costs can be reduced.
The data driving unit 34 is mounted on a flexible circuit board 30 for connecting the liquid crystal panel 1 to a printed circuit board 36, and applies an image signal onto the liquid crystal layer 9 through the data line 5. On the printed circuit board 36, a timing controller and a line are formed.
FIG. 2 is a view schematically showing a structure of the gate driving unit 20. As shown, the gate driving unit 20 is provided with a plurality of shift registers 22. Signals are sequentially produced from the shift registers 22 and applied to the gate lines G1˜Gn. The shift register 22 is connected to a clock generating unit 24, and thus a clock signal generated from the clock generating unit 24 is applied to the shift registers 22. A start voltage is sent to the shift registers 22, and an output signal of the previous shift register is sent to the next shift register as a start voltage after the first shift register.
FIG. 3 is a waveform view showing a start signal S, clock signals C1, C2, C3, and C4 sent to the shift register, and output voltages Vout1 to Voutn generated from the shift register 22. As the start signal S and the clock signals C1, C2, C3, and C4 are respectively sent to each stage of the shift register, the shift register 22 of each stage produces the output signals Vout1 to Voutn thereby to sequentially apply the output signals to gate lines.
The gate driving unit is integrally formed with a liquid crystal panel portion. That is, the shift register 22 is integrally formed on a substrate with a liquid crystal panel portion. Accordingly, a transistor, etc. constituting the shift register 22 is formed by a photolithography like a thin film transistor and acts as a switching device formed at a pixel region of the liquid crystal panel portion. The transistor is generally fabricated by using an amorphous silicon. A gate driving unit to which the shift register having the transistor fabricated by using an amorphous silicon is applied has the following problems.
As output voltages from the shift register 22 are applied to the thin film transistor of the pixel region as scan signals, the thin film transistor is turned on and at the same time, an image signal applied from the data driving unit is charged to a storage capacitor through a channel of the turned-on thin film transistor. That is, during a first period of an output voltage of a rectangular wave form shown in FIG. 3 (1H, that is, the period that a thin film transistor of a liquid crystal panel is turned on or the time that a signal is applied to a pixel), a signal is applied to the liquid crystal layer and a signal is charged to the storage capacitor.
Generally, an amorphous silicon is known to have a low field effect mobility. The low field effect mobility prevents a scan signal applied to the thin film transistor of the pixel region (that is, an output voltage of the shift register) from being a perfect rectangular wave. As shown in FIG. 4, the time of a signal rise and the time of a signal fall are delayed thereby to form a lowered tail region of an ideal rectangular wave. The rectangular wave decreases the turned ON time of the thin film transistor, thereby decreasing an effective time that an image signal is charged to the liquid crystal panel and thus deteriorating a picture quality of the LCD device.
As a resolution of the LCD device increases, the time for charging an image signal is decreased. For example, the time for charging an image signal in one pixel is approximately 60 μsec in case of a QVGA-LCD device. On the contrary, the time for charging an image signal in one pixel is approximately 20 μsec in case of an XGA-LCD device of a high resolution. As the charging time decreases, the lowering of the scan signal due to a low field effect mobility causes an effective charging time to be decreased much more. Accordingly, a picture quality of the LCD device may be degraded in the case of a high resolution device.
In order to solve the problem due to the low field effect mobility, the thin film transistor has to be fabricated to have a very large size (for example, several thousands of μm). However, since a region for forming a gate driving unit is greatly increased, the method was substantially impossible.